834: Dr. S. W. J. Smith on the 



solutions of other acids should fail to yield results of the 

 same kind. 



§ 2. Electrolytic Solution Pressure. — A method of regard- 

 ing the question whether a metal will dissolve in an acid 

 solution has been suggested by Nernst and developed by 

 himself, Ostwald and others. The general idea of this 

 method has become familiar, and provides a means of forming 

 a more suggestive picture of what actually happens than is 

 obtained from purely thermochemical considerations (Cf e. g. 

 Thomsen, ' Thermochemistry,' pp. 349 to 356, &c), un- 

 satisfactory in other ways as well. 



In order to present a consecutive account of the pre- 

 sent experiments, this view is restated below in a form 

 as far as possible free from hypotheses not absolutely 

 necessary*. 



§ 3. A conception of the Interaction of Metals and Acids. — 

 Imagine that a metal M comes suddenly into contact with an 

 air-free solution of an acid HX. It is known that in general 

 equilibrium will be impossible ; a certain quantity of H will 

 be precipitated upon the surface of M and an equivalent 

 quantity of M will dissolve. 



It may be that this interchange will take place only to an 

 indefinitely small amount, but w r e can safely say that no salt 

 MX is absolutely insoluble and that every metal has at least 

 some tendency to go into solution. It is the relative mag- 

 nitude o£ this tendency which is to be regarded as the 

 characteristic variable distinguishing Hg, for example, from 

 metals like Fe and Zn. 



On account of the electrostatic resisting forces which 

 would arise, ions of H cannot escape spontaneously from a 

 solution of HX, nor can ions of M escape from the metal M ; 

 but with M and HX in contact the conditions alter. The 

 assumed tendencies of M and H to spread beyond their 

 original boundaries can now become effective without 

 development of electrostatic resisting forces ; because 

 equivalents of H and M can pass across the common surface 

 of acid and metal without change in the electric charge on 

 either side. Xeutral molecules of H can escape from solution 

 while ions of M enter. 



It is a necessary conclusion from a consideration of this 

 kind (abstract thermodynamics furnishes many other examples) 

 that solution of the metal and precipitation of hydrogen 

 must begin at the interface (assuming no other change pos- 

 sible), whether accompanied by loss or gain of heat from the 

 * Cf. Nernst, ' Theoretical Chemistry,' 2nd Engl. edit. p. 724. 



